This invention relates generally to pressure compensated load responsive flow control valves of direction control type, which in control of a load, while using a control load pressure sensing passage, automatically maintain pump discharge pressure at a level higher, by a constant pressure differential, than the pressure required by the controlled load, by bypassing excess pump flow to system reservoir. Such a control valve disclosed in U.S. Pat. No. 3,488,953 dated Jan. 13, 1970, although effective in control of a single positive load at a time, cannot simultaneously control multiple positive loads.
This disadvantage is overcome by control valve disclosed in my U.S. Pat. No. 3,882,896 and my pending patent application Ser. No. 522,324 filed Nov. 8, 1974, entitled "Load Responsive Fluid Control Valves", in which individual check valves, in load pressure sensing passages, permit phasing pressure signals of only the highest system load to the differential bypass control of the flow control valve, while isolating pressure signals from other loads. Those valves, although effective in control of multiple positive loads, suffer from a number of disadvantages. Because of the large cross sectional area of the differential bypass valve and its long control stroke, a comparatively large volume of fluid is required to operate it. Therefore small diameter and length of load pressure sensing passages, through which the fluid needed for displacement of the differential bypass valve must pass, limit the response of the valve control and tend to attenuate the control signal.
The response of the differential bypass valve is also adversely affected by another factor. Since the displacement of fluid, caused by the movement of the differential bypass valve in one direction tends to close the check valves in control of load sensing passages, isolating the control space filled with fluid, a constant path of leakage must be provided between the load sensing signal circuit and the system reservoir. This control leakage is usually obtained by providing an orifice between load sensing circuit and system reservoir. Since flow through the orifice is proportional to the square root of pressure differential acting across it, and since flow through the orifice determines response of the differential bypass valve in one direction, an acceptable response of control at low system pressure results in high leakage losses through the control orifice at high system pressure. This not only adversely affects the efficiency of the control valve, but also, since all of the increased leakage flow must be supplied through load pressure sensing passages, further attenuates the control signal.